Replacing OA2 tubes with Zenier Diodes in Preamp

The other advantage in keeping the 0A2/6EM7 combination is the fact that they act as a soft start circuit. By replacing the 0A2's with a zener diode you will put voltage directly on the 6EM7 before it's had a chance to start conducting, usually 10-15 seconds. This could limit the life of the 6EM7. If you are trying to tighten the bass performance of this product, I suggest you try something else and keep the tubes in the power supply.

G. Garfield

Mostly cons. Zeners can be very noisy and can tend to dry up the sound when compared to tube references. You may lose that little bit of "give" in the base response. Ditto for replacing the tube regulator with solid state.